1. Field of the Invention
The present invention relates to a regenerative drive control and more particularly to a regenerative drive control including a negative converter bridge which powers a load having inductance and resistance and wherein a circulating path is provided through the negative converter bridge for inductive current in the load to enable the inductive current to exponentially decay toward zero upon the occurrence of a fault when the normal sequential gating of the converter SCR's is inhibited.
Regenerative drive controls are well known in the art. In known regenerative drive controls which are applied to SCR phase control converter bridges operating in the negative current mode, problems are associated as a result of high current which results when the converter SCR gate pulses are inhibited while the bridge is operating into an inductive load at high continuous negative current and high average output voltage. The prior art devices are subject to loss of fuses or destruction of SCR's or other circuit components of the system as a result of the high current. The present invention prevents the loss of fuses or SCR's by diverting current to a circulating path before the current increases to catastrophic levels as a result of a fault condition.
2. Prior Art
Converters have commonly been used with DC power lines for high voltage power transmissions. Eckstrom U.S. Pat. No. 3,835,365, Machida U.S. Pat. No. 3,636,431 and Gusakowsky U.S. Pat. No. 3,609,508 all disclose examples of such a system. Ekstrom and other known prior art do not apply the converter systems to industrial drive applications where electrical power is converted to mechanical power. Such applications result in different considerations for converter system design than converter systems for high voltage power transmission due to the potential regenerating capabilities of the mechanical load. In addition, the prior art does not react fast enough to prevent a regenerative margin commutation failure as described hereinbelow. The prior art also does not disclose converter fault protection in a regenerative drive control for a negative converter bridge wherein a fast response is provided to prevent regenerative margin commutation failure and provide a by-pass path through the negative converter bridge for the inductive energy of the load.
Other fault protection systems are disclosed in the Overzet U.S. Pat. No. 4,139,885 and the Morris U.S. Pat. No. 4,309,735. Both Morris and Overzet disclose forced commutation systems which buck or force commutate the current in the converter bridge. Morris utilizes a capacitor 20 to buck current and Overzet force commutates the converter bridge using additional power components SCR 47, capacitor 48 and the charging circuit at 50. The forced commutation scheme disclosed in Overzet and Morris is undesirable because it adds components to the power circuit which can reduce reliability and increase the cost and size of the unit. The present invention overcomes the above noted problems by providing a circulating path through a pair of simultaneously conducting switching devices in a negative converter bridge for the inductive current of the load prior to the build up of the current to a destructive level to thereby enable the inductive current to exponentially decay toward zero upon the occurrence of a fault.
In a regenerative drive control where current is reversed to affect regeneration, two converter bridges are required. One bridge is called the positive converter because it can only conduct positive load current. In the positive bridge the output terminal formed by the connection of the three SCR cathodes is connected to the positive terminal of the load and the output terminal formed by the connection of the three SCR anodes is connected to the negative terminal of the load. The second bridge required is defined as the negative converter bridge since it only conducts negative (regenerative) load current. In the negative converter the SCR's are reversed in each leg of the bridge so that the SCR common anode output terminal is connected to the positive terminal of the load and the SCR common cathode output terminal is connected to the negative terminal of the load. [The foregoing definition of positive and negative converters is derived from the description of a dual converter on pages 111-113 of Thyrister Phase-Controlled Converters and Cycloconverters by Brian R. Pelly, published by Wiley Interscience.]
With respect to operation of converter bridges, the positive converter will be line commutated (also known as natural commutation) for all rated levels of SCR current and DC output voltage; that is to say that commutation of a conducting SCR will result if either (1) the next SCR in the gating sequence is gated or (2) all gating to the bridge is inhibited. For condition (1) the polarity of the AC input voltage at the time of gating is in the direction to commutate the current away from the conducting SCR and into the gated SCR. In condition (2) the AC input voltage is such that it forces the current in the conducting SCR's to zero thus commutating them. In either case, device commutation is not in question and is line commutated.
Line commutation is not guaranteed in the case of a negative converter bridge. This is the most significant difference between the positive and negative converter bridges. In a negative converter bridge there exists a level of continuous negative load current and positive bus voltage beyond which the conducting SCR will not be line commutated if gating to the bridge SCR's is lost or inhibited. Above these levels there is insufficient volt-seconds available in the portion of the line-to-line waveform above the bus voltage to force the load current to zero, thus commutating the SCR, according to the equation ##EQU1## When the line-to-line voltage becomes less than the bus voltage at time t.sub.2, the rate of change of load current reverses and the current is forced to destructive levels by the large voltage difference that develops between the bus voltage and the conducting AC line. The peak voltage difference is greater than 1200 V given a +600 V bus voltage and would result in a peak perspective current well beyond the negative converter bridge SCR's rating.
It is important to understand that a critical aspect of the present invention is that the gating of the switchable devices to form a circulating path must be simultaneously accompanied by the deenergizing of the regenerating load to prevent the sustained generation of a high level of negative current by the load. Without the deenergizing of the load, the gating of the switchable devices to form a circulating path would result in levels of negative circulating current which would destroy said devices.